While Ziegler-Natta catalysts are a mainstay for polyolefin manufacture, single-site (metallocene and non-metallocene) catalysts represent the industry's future. These catalysts are often more reactive than Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of α-olefin comonomers, lower polymer density, controlled content and distribution of long-chain branching, and modified melt rheology and relaxation characteristics.
Traditional metallocenes incorporate one or more cyclopentadienyl (Cp) or Cp-like anionic ligands such as indenyl, fluorenyl, or the like, that donate pi-electrons to the transition metal. Non-metallocene single-site catalysts, including ones that capitalize on the chelate effect, have evolved more recently. Examples are 8-quinolinoxy or 2-pyridinoxy ligands (see U.S. Pat. No. 5,637,660) and the bidentate bisimines of Brookhart (see Chem. Rev. 100 (2000) 1169).
Recently, we described chelating bicyclic dianionic ligands useful for olefin polymerization catalysts (see copending application Ser. No. 09/907,180, filed Jul. 17, 2001). In these complexes, one ligand chelates to the metal through two separate allylic anions, each of which is a 4-pi electron donor. Molecular modeling calculations indicate that the steric and electronic environments of these ligands are comparable to those of conventional metallocene ligands. Their “open architecture” suggests that comonomer incorporation will be facile. Triquinane or other tricyclic dianionic ligands are not disclosed.
We also described earlier the use of Diels-Alder and photo-chemical [2+2] cycloaddition reactions in tandem to make “caged diimide” complexes (see copending application Ser. No. 09/691,285, filed Oct. 18, 2000). Conversion of a caged diketone to the corresponding diimine, followed by preparation of a transition metal complex incorporating the neutral diimine ligand affords complexes useful for olefin polymerization.
The tandem strategy was used by G. Mehta et al. (J. Am. Chem. Soc. 108 (1986) 3443) in a remarkable route to triquinane natural products such as (±)-hirsutene and (±)-capnellene. The key to assembling these skeletons efficiently was recognizing that the cis,syn,cis-triquinane skeleton is available in three steps in near-quantitative yield (>80% overall) from inexpensive starting materials (p-benzoquinone and cyclopentadienes), and essentially no chemical reagents other than a reaction solvent (in addition to heat, and light): In the example shown above, simple thermolysis of pentacyclic diketones 3 and 4 gave only the cis,syn,cis-triquinane bis(enone)s 5 and 6. After these pivotal, elegant steps, Mehta further elaborated the bis(enone)s to make the desired natural product, capnellene.
The polyolefins industry continues to need new polymerization catalysts. Unfortunately, the organometallic complexes are becoming increasingly complicated and more expensive to manufacture. The industry would benefit from ways to achieve a high level of molecular complexity in relatively few synthetic steps. The accessibility of a host of interesting triquinane skeletons invites polyolefin makers to explore their applicability outside the realm of natural products synthesis. Catalysts with advantages such as higher activity and better control over polyolefin properties are within reach. Ideally, these catalysts would avoid the all-too-common, low-yield, multi-step syntheses from expensive, hard-to-handle starting materials and reagents.